1. Field of the Invention
The present invention generally relates to semiconductor manufacturing process. More particularly, the present invention pertains to methods and apparatuses which use precious gases, like deuterium, during a high pressure annealing process of semiconductor manufacturing.
2. Description of the Related Art
During the semiconductor manufacturing process, various different thermal treatments are performed on a semiconductor wafer, for example, during or following oxidation, nitridation, silicidation, ion implantation, and chemical vapor deposition processes, to create the integrated circuits on the semiconductor wafer.
Key determining factors for effective fabrication of integrated circuits not only include the process temperature, but also the processing time and the concentration of a particular gas or a mixture of gases used for a particular application or treatment. These three factors are generally considered as independent variables which determine the efficiency of the processing. For example, by increasing the process temperature while keeping the gas concentration constant, the process efficiency will improve. Similarly, by increasing the gas concentration at the same temperature, the process efficiency can be improved. It should be noted that exposure of semiconductor wafers, or more precisely integrated circuits, to excessive heat generally degrades the quality of the integrated circuits, in an irreversible and cumulative way. This is, in part, due to the diffusion of various carriers and ions implanted on the wafer, whose rate increases, typically superlinearly, with temperature. Each integrated circuit has an acceptable limit of total thermal exposure during the whole manufacturing process, which is referred to as the circuit's thermal budget in the related art.
As the technology and device structure approaches the nanometer scale, the limited thermal budget requirement demands higher concentration of the processing gas. Annealing wafers in a forming gas containing diatomic hydrogen, typically following fabrication but before encapsulation or other packaging steps, has been widely used for repairing various process induced damages during the semiconductor fabrication process as well as for sintering process, which is referred to as hydrogen passivation in the art. The annealing or forming gas generally incorporates approximately 2% to 10% hydrogen (H2) with the remainder being inert gas such as nitrogen (N2).
Recently, however, many researchers have reported that pure (100%) deuterium anneal improves the device characteristics and performance such as hot carrier reliability, transistor lifetime, and reduction of dangling bonds and unwanted charge carriers. Improvement of device lifetime increases the transconductance (speed performance) of the device. As the device technology and structure move to the sophistication of the so-called “nanometer technology”, new high pressure application technologies require use of other gases such as fluorine (F2), ammonia (NH3), and chlorine (Cl2), which can be highly reactive or toxic. The forming gas (partial pressure) anneal and/or pure H2 or D2 anneal has been generally done at a temperature range above 450° C., and higher temperature tends to result in better performance. However, as the device scale reaches 28 nm or below, the limited thermal budget after first metallization requires annealing temperatures at or below 400° C., thus potentially diminishing the hydrogen annealing benefit on semiconductor device performance.
As an alternative, hydrogen or deuterium high pressure annealing can result in excellent performance and improvement. Particularly, hydrogen and/or deuterium annealing of high-K gate dielectric device showed significant performance improvement in charge reduction, dangling bond reduction, and increase of transconductance. This finding has been disclosed, for example, in the U.S. Pat. No. 6,913,961 and U.S. Pat. No. 6,833,306. This improvement is very significant for the manufacturing process of integrated circuit devices using high-K gate dielectric for the next several generations of semiconductor device technology.
High pressure annealing, in particular, in the hydrogen (H2) or deuterium (D2) environment can improve performance of semiconductor devices. This finding has been disclosed, for example, in the U.S. Pat. No. 8,481,123. In that patent, titled: Method For High Pressure Gas Annealing, various embodiments are disclosed to anneal a silicon substrate wafer in a high pressure environment. As disclosed in that patent, in a high pressure annealing process, high pressure hydrogen or deuterium gas is used in various annealing processes, such as high-K gate dielectric process anneal, post-metallization sintering anneal, and forming gas anneal. The use of high pressure gas can significantly improve the device performance. For example, it could increase the device's lifetime and its transconductance, and it can decrease the number of dangling bonds. One of the main advantages of the high pressure gas annealing is that these improvements in the device performance can be achieved with a reduced thermal budget cost at a given temperature and/or a given processing time, which is an essential requirement for the advanced device technology.
It is known that one of the main advantages of the high pressure technology is the increase of the reaction rate by effectively increasing the gas concentration at high pressure. By increasing the pressure of the processing gas, the density of the processing gas will increase. The gas density increases roughly linearly as the pressure increases. For example, if pure 100% hydrogen or deuterium is processed in 5 atm high pressure condition, the actual amount of hydrogen or deuterium gas that semiconductor silicon is exposed to is 5 times the concentration of the original (100%) hydrogen or deuterium gas at the atmospheric pressure. In the case of partial pressure conditions, if the hydrogen or deuterium concentration is 20% and the silicon wafer is processed at 5 atm pressure, then the silicon wafer is effectively exposed to the equivalent of 100% hydrogen or deuterium at atmospheric pressure. Likewise, processing with 20% hydrogen or deuterium gas at 20 atm will be roughly equivalent to 4 times of the processing result with the pure (100%) hydrogen or deuterium gas at 1 atm.
By increasing the pressure of the process gas, it is possible to reduce both the processing temperature and the process time. As the thermal budget limitation reaches the “extreme limit level,” and as the device technology reaches the 28 nm range, high pressure processing becomes a viable solution which meets or exceeds many thermal processing requirements in the semiconductor fabrication technology. The high pressure processing can provide the following benefits with respect to the three aforementioned process parameters; process time reduction, process temperature reduction, and process gas concentration reduction. (1) By increasing pressure, the process temperature can be reduced while maintaining the gas concentration and process time unchanged in order to obtain equivalent or similar process results. (2) By increasing pressure, the process time can be reduced significantly while keeping other parameters of temperature and gas concentration unchanged in order to obtain equivalent or similar process results. (3) By increasing pressure, the process gas concentration can be reduced while maintaining the time and temperature parameters unchanged in order to obtain equivalent or similar process results.
Application of high pressure hydrogen/deuterium process anneal to high-K gate dielectric process anneal, post-metallization sintering anneal, and forming gas anneal in the semiconductor fabrication could achieve a significant improvement in the device performance, for example in terms of increased device lifetime, enhanced transconductance, and reduced number of dangling bonds, and also achieve significant process thermal budget improvement at a given processing temperature and processing time, which is an essential requirement for the advanced device technology.
As described in U.S. Pat. No. 8,481,123, the gas from the outer chamber is released at the same time and mixed with the hydrogen/deuterium gas or other toxic or flammable gas from the inner chamber. Another inert gas such as nitrogen is added during the venting process thereby further reducing the concentration of the reactive gas exhausted to the atmosphere from the annealing vessel. After the process is completed and the gases used for various purposes are depressurized, any remaining residual gas trapped in the annealing chamber are safely removed by purging extra nitrogen flow near, or around, the exhaust valves or pipes of the annealing vessel before discharging the remaining gases into the atmosphere. This is done to avoid direct exposure of concentrated hydrogen or deuterium with the atmosphere, to prevent a potentially dangerous condition.
The high pressure annealing processing unit, as described in U.S. Pat. No. 8,481,123, comprises a vertical high pressure processing system, as illustrated in FIG. 1. According to that invention, the annealing vessel has a dual chamber structure, comprising an inner chamber and an outer chamber, and a reactive gas, which may be flammable, toxic, or otherwise dangerous, is confined in the inner chamber. The inner chamber is then protected by the external pressure exerted by another gas contained in the outer chamber. This design provides a buffer zone in case where there is a leakage of the processing gas from the inner processing chamber, and hence it provides, among other things, two main benefits: It dilutes the potentially dangerous gas leaked from the inner chamber, and it prevents the leaked gas from directly releasing into the air. In certain embodiments, more than one outer chambers are used to provide multiple layers, or buffer zones, of protection. The main external vessel, or the outer chamber, shown in the figure comprises three components, top 37, body 39, and bottom 38. In some embodiments, these external vessel components are made of type 316 stainless steel material that has high stress point to pressure. The vessel top 37 is normally attached to the main vessel body 39 by screws, and the vessel bottom 38 is attached to the main vessel 39 using a breech door locking 40, which is also made of type 316 stainless steel in some embodiments. In this exemplary design, the vessel bottom is separated from the main vessel when the vessel door opens for loading and unloading.
Inside the main vessel, there is a 4-zone main heater 34 that controls each heater zone independently. The heater elements 34 are insulated from the vessel wall by an insulator 33. There is also a 2-zone plug heater 24 on top of the bottom component of the vessel 38 in this embodiment, which can heat the wafer holder or wafer boat 22 from the bottom. The wafer boat holds one or more semiconductor wafers 23, and in some embodiments, it is made of quartz. The external main vessel has cooling water lines 31 to prevent the vessel from overheating by the heater 34 inside the vessel beyond the safety temperature. Around the plug heater 24, quartz cap 27 is placed, and it has quartz helix around the plug heater that will heat the incoming process gas to the process temperature. The process gas is introduced into the inner processing chamber, or tube, 21 via a gas injector 26, which pressurizes the tube. The inner process chamber is made of non-metallic materials such as quartz and the outer chamber is made of metals or metallic alloys such as stainless steel.
In other embodiments, both chambers are made of metallic materials with high melting points. The inner chamber 21 divides the space in the vessel into two regions, and the gases in these two regions can be completely isolated and they can have different pressures. The gas pressure inside the process chamber, indicated as 20 in the figure, is called a tube pressure and the pressure outside the inner chamber, indicated as 30 in the figure, is called a shell pressure. The outer shell chamber is pressurized by gas typically different from the processing gas, which may be highly reactive, flammable, or otherwise dangerous. In some embodiments inert gas such as nitrogen is used for this purpose. Nitrogen is introduced into the outer chamber via a shell nitrogen injector 50 in the exemplary embodiment shown in the figure. The figure also shows two chill plates, top 32 and bottom 28, which are used to protect components in the temperature protected areas above the top chill plate 32 and below the bottom chill plate 28 from excessive heat. The shell pressure area inside the outer chamber and the tube pressure area inside the processing chamber are separated and sealed by O-rings 25. O-rings 36 are also used to hold the shell pressure by preventing the inert shell gas from leaking from the main vessel to the outside atmosphere.
Equalizing, or near-equalizing, pressures of the shell nitrogen 30 and the tube hydrogen 20 will maintain the integrity of the quartz tube from collapsing, either inward or outward. When the tube is fully pressurized by hydrogen/deuterium or other processing gas to the designated pressure level, the shell is also pressurized by nitrogen or other inert gas to the same or comparable pressure level.
When the high pressure processing is completed, the tube pressure 20 will be released via de-pressurizing exhaust 29, and the shell pressure 30 will be released via shell pressure exhaust 35, which are controlled by a pressure control valve 41. Both the shell pressure and the tube pressure is controlled by the same pressure control valve or a set of valves. When the pressure control valve 41 releases the pressure, the nitrogen in the shell and hydrogen or other process gas in the tube are simultaneously released to the exhaust. The exhaust gases are mixed, and this effectively dilutes the processing gas such as hydrogen with nitrogen and also maintains the pressure differential between the two chambers within a desired range. In the exemplary embodiment shown in the figure, where the volume of the outer chamber is three times that of the inner chamber, the concentration of the processing gas from the inner chamber becomes diluted to the one-third level of its original concentration. For example, when a forming gas with 30% hydrogen has been used during the annealing, the hydrogen concentration in the exhaust will be around 10%. The pressure of the gases is maintained with the help of a computing device associated with the high pressure processing unit. Examples of a computing device can be a programmable logic, control, and ASIC control, or any computing device that can be integrated and/or associated with such a system, as known to one of ordinary skill in the art. Further, it will be appreciated that pressure sensors within both the inner and outer chambers may be coupled to a computer which provides the control described herein, and this control may be implemented through a software program executing on the computer.
When the pressure control valve 41 opens, the pressures of both chambers are simultaneously released while the gases of nitrogen and hydrogen are still under high pressure. Hydrogen, though diluted by nitrogen from the shell, should not be exposed to the atmosphere. Any defects in the exhaust pipe, typically made of stainless steel, will release hydrogen into the atmosphere. In order to prevent such unwanted leak from defects in the stainless steel pipe, the exhaust line stainless steel pipe, 42 in FIG. 1, is made of double-walled stainless steel in some embodiments of the present invention. In the double-walled stainless steel construction, if the first or inner gas pipe experiences a defect and the gas leaks, the second or external protective pipe will contain any leaked hydrogen in the pipe. Thus the likelihood of the gas leak directly into the atmosphere is significantly reduced. The hydrogen, diluted by the shell nitrogen, flows to the dilution tank 43 via the double-walled exhaust line 42 to be further diluted prior to moving to the hydrogen/deuterium burning scrubber 45 via another double-walled stainless steel line 44. After the scrubber burns the hydrogen and any flammable gas in the exhaust, it will release the burnt residue into the atmosphere, indicated by the arrow 53 in the figure. The exhaust vent line will most likely have water condensation inside the line, particularly if the scrubber is not used, due to the back streaming air from the atmosphere, which typically has much lower temperature than the exhaust gas. The condensation may react with hydrogen since water (H2O) contains oxygen. This could be a source of safety problem. In order to prevent the water condensation and also to increase dilution of venting hydrogen/deuterium, additional nitrogen is injected in the exhaust vent line in some embodiments. FIG. 1 shows a nitrogen injection line 56, which is connected to the exhaust vent line immediately after the exhaust vent valve, and this injection line 56 serves as a constant source of nitrogen to guarantee a constant overflow of a gas from the outlet of the scrubber 45. According to at least one embodiment, low flow of nitrogen is maintained during the normal operation in order to prevent any condensation in the vent line and to maintain an always outward flow of nitrogen from the scrubber 45. During the chamber depressurization, the nitrogen flow may be increased in order to further dilute the venting hydrogen/deuterium or any other potentially dangerous processing gas exhausted from the annealing vessel.
FIG. 1 shows a source of high pressure hydrogen or deuterium as the incoming processing gas via canister 49. The incoming processing gas flows into the gas control panel or cabinet 46 through gas lines 54 and 48, and it is injected into the processing chamber 21 through gas pipe 51 and through the gas injector 26 (the gas line between the pipe 51 and the injector 26 is not explicitly shown in the figure). The incoming hydrogen or deuterium gas may be 100% pure, and the pressure is typically 500 PSI at minimum, and hence the incoming gas lines, for example, 54, 48, and 51, and various parts around the gas canister or pump can be one of the most dangerous areas in the high pressure system. The system also includes a H2/D2 gas panel, where all the gas control components (not shown in the figure) are installed. An H2/D2 detector sensor is installed inside the control panel. Thus, the presence of the hydrogen or deuterium sensor enables the system to distinguish between the gasses used to anneal the substrate wafer. The high pressure annealing processing system is not limited to any particular processing gases, and any type of gas may be used based on application requirements.
However, when a precious gas (e.g., deuterium) is used as the annealing gas, the high pressure annealing processing unit would safely discard the annealing gas from the exhaust. Thus, the precious gas is discarded and is not reused. No system exists that could safely extract the used precious annealing gas (e.g., deuterium) from the exhaust of the annealing process for reuse.